Many systems for industrial and residential control of environmental temperatures employ continuous vapor cycle sequences, which have been widely employed and have subsequently evolved into many different configurations. Typically, such systems continuously cycle a two-phase fluid, such as a refrigerant with a suitable evaporation point, by first pressurizing the refrigerant into a hot gas phase and then condensing the refrigerant to a liquid phase of suitable enthalpy for subsequent controlled expansion to a lower target temperature. Thus cooled, the refrigerant is passed in heat transfer relation to a thermal load, usually by employing an inert heat transfer fluid, and the two-phase refrigerant is thereafter cycled back within a closed loop for repressurization and subsequent condensation.
A meaningful departure from this approach to industrial and residential temperature control is described in recently issued patents U.S. Pat. No. 7,178,353 and U.S. Pat. No. 7,415,835 to inventors Kenneth W. Cowans et al. This departure is directed to a novel temperature control system which combines flows of refrigerant in a hot gas pressurized mode with the same refrigerant in an expanded vapor/liquid mode. The system combines some expanded refrigerant flow with a suitable proportion of pressurized hot gas in a closed circuit vapor-cycle refrigeration system. The combined refrigerant stream generated can exchange thermal energy directly with a load, as in a heat exchanger (HEX). Such systems offer substantial benefits in improving heat transfer efficiency and economy and in enabling rapid and precise temperature level changes. Since they require no intermediate coolant and the pressure can be varied rapidly, this approach, which for succinctness has sometimes been termed TDSF for “Transfer Direct of Saturated Fluids” offers distinct operative and economic advantages for many temperature control applications.
Many different improvements involving special thermal exchanges between different fluids have been offered for use in the broad field of temperature control systems. A patent to Goth et al, U.S. Pat. No. 6,644,048 dated Mar. 10, 2003 for example, proposes a scheme for modifying a refrigerant used directly in heat exchange relation to a thermal process, by employing a controlled solenoid valve to inject bursts of hot pressurized gas into a cold refrigerant. This is done to assist in transitions from colder temperature level to higher temperature levels, such as for startup, cleaning, and other purposes. The Goth et al patent does not teach control at a selected or variable temperature level, and is concerned with increasing the temperature level by adding one or more bursts of hot gas for the purpose of avoiding water condensing on sensitive electronic circuits. It accordingly is not useful as a basis for generating precisely controlled temperature levels across a range of temperatures.
Other patents propose the use of special HEXs for establishing special effects. For example, U.S. Pat. No. 5,245,833 to V. C. Mei et al, entitled “Liquid Over-Feeding Air Conditioning System and Method” discloses a “liquid over-feeding” operation in which heat is exchanged in an accumulator-heat exchanger. This exchange is between a hot liquid refrigerant, and a cooler output refrigerant, after which the refrigerant is expanded for cooling before being applied to the evaporative load. This sequence subcools the refrigerant to allow more of the evaporator surface to be used for cooling. A later variant of this approach is disclosed in U.S. Pat. No. 5,622,055, entitled “Liquid Over-Feeding Refrigeration System and Method with Integrated Accumulator-Expander-Heat Exchanger” by V. C. Mei et al. This variant improves heat transfer by subcooling the refrigerant to a lower level using a capillary tubing immersed in a pool of liquid refrigerant. This approach requires a unified vapor cycle configuration, with specially modified evaporator and exchangers and is not readily suitable for modifying existing compressor-condenser systems so as to improve efficiency and save energy.
Different approaches to energy saving have also been disclosed by the same inventor teams in two heat pump patents, namely U.S. Pat. No. 5,845,502 to F. C. Chen et al entitled “Heat Pump for an Improved Defrost System” and U.S. Pat. No. 6,233,958 to V. C. Mei et al entitled “Heat Pump Water Heater and Method of Making the Same”. The expedients used are primarily of interest to the heat pump approach and do not suggest how thermal efficiency improvements can feasibly be effected by modifying existing vapor cycle system for energy conservation.
As energy demands have continued to increase and limitations on the use of energy sources have continued to be encountered, it has become increasingly evident that much can be gained by improving the efficiency of present systems. Even relatively modest improvements in the energy usage of air conditioning systems, for example, can pay substantial dividends over the long periods of use that such systems undergo. Accordingly, any economically realizable modification of the thermodynamics of basic vapor cycle sequence that provides meaningful efficiency improvement, reductions in energy costs, or both, can have broad consequences for vapor cycle systems.
In accordance with the Cowans et al patents previously alluded to, substantial benefits are in fact gained because of the inherent advantages of direct transfer of thermal energy using a saturated fluid, (the TDSF approach). Such systems employ a vapor cycle configuration in which the pressure-enthalpy interactions in the cycle are inherently more complex because they use, in integrated fashion, both hot gas and expanded vapor mixed with liquid. Because of asymmetries between the thermal exchange characteristics of these two flowing media, instabilities and imprecision can arise in temperature control applications especially when corrections are small and loads are low. Achieving improvements in internal efficiency in TDSF-types systems can be of benefit, but imposes special problems.